A solar eclipse is a happy scientific coincidence. As seen from Earth, the Moon and Sun are the same size in the sky. So when the geometry is right, the Moon can cover up all or most of the Sun’s disk.

An eclipse of the “most” variety is coming up on Sunday afternoon[2] for parts of the western United States. It’s called an “annular” eclipse because the Moon won’t cover up all of the Sun. Instead, it’ll leave a thin but bright ring of sunlight around the Moon.

Solar eclipses occur when the new Moon crosses directly between Sun and Earth. They don’t happen at every new Moon, though, because the Moon’s orbit around Earth is tilted with respect to Earth’s orbit around the Sun. So most months, the Moon passes a little above or below the Sun as seen from Earth.

But two or three times a year, on average, the geometry is just right, creating an eclipse. Sometimes it’s total, sometimes it’s annular, and sometimes it’s just partial — the Moon obscures only a part of the Sun’s disk.

The “coincidence” part comes in because of the relative sizes of the Moon and Sun in the sky. The Sun is 400 times wider than the Moon, but it’s also 400 times farther, so they take up the same amount of sky.

The Moon is moving farther from Earth, though — at about an inch-and-a-half a year. So in a few hundred million years the Moon will be too far away to ever completely cover the Sun — and the spectacle of a total solar eclipse will disappear.